In contrast, here's what can happen when the advance is shortened by limiting how far out the weights can move.
Initial is set at 12.5* 700 rpm.
This will be a problem for street use or other applications where vacuum advance is wanted.
(Obviously there's no slew with points. Pointing out this disadvantage of having an advance 'all in' early when used with electronic ignitions.)

You lost me. You are throwing a lot of data out without the analytics. Your also saying the distributor never moves and you set initial by welding inside. Assuming everything else in the distributor is perfect for your new set up of higher initial and less mech adv. Which it is not. I also think some of your assumptions about vacuum are wrong. Was this just a linear computer equation you used, or a generated graph from actual experimentation?

I just measured several Prestolite dual point cams. Everything from the Hemi Cap 15deg distributor adv (0 initial), to a tach drive race cam, and several intermediate ones. The inside of the slot is at the exact same point at about 7/16" from collar. The difference is the length moving outward. I do not have as many Chrysler cams loose, but did check two 8s versus a 11 and again the inside distance from collar was the same. In this case 3/8".

Having studied the differences in shaft tip clocking, and cam stop flat location, it was all about housing orientation to ensure vacuum cannister, oiling port access was not obstructed by manifold, fuel lines, etc... Watching a distributor being fitted on a hemi with cross ram manifold with vacuum advance (street motor). You had limited amount of rotation for setting initial timing. In fact one Chrysler type would not work. When I inspected why, the tip of shaft was clocked slightly different around 10 deg (Chinese knock off). Would not have mattered in a 440 or stock setup. This prevented getting intitial high enough because vacuum cannister hit manifold.

As said in beginning you can do this either way, but that does not mean your tuned right. Weld up to limit mech adv. Set initial by housing rotation, set advance curve by spring choice and tension (adjustable).

Sure, the weights cut a curve, but the rotation is small compared to 360 degree circle. So it can be treated as linear. Well within a tolerance of the mechanical play, bearing clearance, spring rate etc.....

regarding the accuracy of the factory plate/cams I've found no more than an half of a distributor degree difference in actual degrees vs what's stamped. for instance a 9 degree plate may be 8.5-9 degrees. never enough to be any kind of "make it or break it". I keep it fairly simple but i'm fortunate enough to have a variety of factory parts and springs. some 5th grade math and a timing tape are your best friends.

I'll see if I can clarify.
The other way. That's the analytics with an example that was handy for me to upload. LOL.

Seriously. For the purpose of showing the effect on the timing, yes that is what this first graph with the black line shows.

We could just look at it as advance. That's often harder for people to picture how that effects the timing curve.
This example is about changing the advance so more initial timing can be used without changing the middle and top end timing.

The next graphs were illustrating how the curve would shift if the initial timing desired (12*) was set at 700rpm.



There is no equations here at all. Those are the timing and advance specifications from the 1968 Plymouth Service Manual. Anything within those lines would have been acceptable. I'm assuming the top or maximum timing for the examples where I sketch in the result of a change. Its not the real curve. Its just four points connected so its easier to see.


That's interesting. I thought I'd seen some variation in distance from collar with the Chrysler cams, but not much. I don't have any points cams to check. Am verious as to the pre-68 non-CAP vs the later ones for the same engines.
One thing certainly seen is angling of the slots in some of the longer advances.


Yes.I agree. If you have a selection of springs, or can wind your own, then it doesn't matter. But the heavy long looped spring, which can be really important, is really difficult to find if you don't have a collection already. So that's why for most folks, the inside is easier approach.

Now you lost me here. LOL. If you're describing the arc of the weights, and the changing angle of the spring, the design is pretty good because it helps to compensate for the centripital force. Some of the aftermarket designs really fail to do that.

Agree that it would be best for the OP, Challenger, to measure timing from idle to as high as he feels comfortable.

My comments about the vacuum advance were about effect of not slowing the rate mechanical advance before 2000 rpm.
Its not hard to reclock distributor to change where the vacuum canister points on a small block. No disagreement that is something to consider.
No idea what engine Challenger is working with.

When vacuum advance is used with a mechanical timing curve like shown below, the engine will ping under light to moderate throttle in the 2000 - 3000 rpm range.
Been there done that.

If the counter weight was locked not to rotate, the pin on the weight that sits in the slot rotates in a small circular motion if viewed from above. As the weight moves outward that pin moves in a curved motion to the new position (not linearly) and makes a larger dia circle in motion. At full advance it is tracing the largest circle. The force that pin puts on the cam to make it rotate is not perpendicular to the slot, rather angular as it curves outward. Lower force applied in the direction of rotation. A cam with an angular slot actually changes that force to perpendicular against the cam slot. Which means it takes less force to move the cam, which means the springs can be heavier and still get the same angular rotation at the same RPM as straight slot which has a lighter spring. If the spring is the same on both, the angular cam will rotate more at the same RPM and get advance in faster.

So besides effecting how much rotation is allowed because of the angular slot, it also effects the force on the cam.

I was confused on your slew rate comment.

We are making this more complicated then needed, and clearly folks think one is better then the other.

I never thought some one was doing this welding up to tune all aspects in one shot. Set initial timing, effect spring rate, limit mechanical. But I get it.

i have welded slots for years. it's easy if one clamps a piece of flat copper under the plate before welding. the weld doesn't stick to the copper, so after a quick clean up of the slots to match the length you want, you are good to go.
i did buy one of those plates, but have yet to try it.
my biggest hurdle in factory distributor modification, is getting the shaft retaining clip out from under the rotor seat.
i finally made a tool for that by welding a couple of pieces of tiny tubing that fit the wire size of the retaining clip onto a pair of small, seldom used pliers. that works great !

I use a thin bladed pair of needle nose pliers to remove them by pulling them straight out, I do have to compress them back so they will work again

2 small tip flat screw drivers and spread clip while gentley lifting cam up. Or one small long needle nose and flat tip. Grab one end, and hold other with flat tip.